The p63 gene encodes several transcription factors that are required for normal development and homeostasis of the skin and its appendages. Although the role for p63 in the epidermis has been well-established, the role for p63 in hair follicles (HF) has remained largely elusive. A role for p63 in HF biology has been suggested based on the observation that hair shaft abnormalities develop in patients with ectodermal dysplasias caused by p63 mutations. However, the pathological mechanisms that lead to these hair abnormalities have not been elucidated. Based on our preliminary findings, we propose that p63 functions in HF stem cells of the bulge. In Specific Aim 1, we will test this hypothesis by ablating p63 from bulge stem cells in genetically engineered mice. Analysis of these mice will define whether p63 functions in bulge stem cell survival, renewal, or activation. Reduced p63 expression has also been observed in human skin squamous cell carcinomas (SCCs), a tumor type that originates from bulge stem cells. However, the functional significance of this observation is not known. Our preliminary findings indicate that a general downregulation of p63 in the epidermis and hair follicles facilitates skin SCC development. In Specific Aim 2, we will further define the role of p63 loss during skin SCC tumorigenesis using genetically engineered mice. We will determine whether ablating p63 from bulge stem cells, in the presence of oncogene activation, results in increased skin tumor susceptibility. Further, we will determine whether ablating p63 from established tumors leads to accelerated tumor progression. Collectively, these studies will provide novel insights into the role of p63 in bulge stem cells under homeostatic and pathological conditions. These studies will have implications for the treatment of hair abnormalities in patients with p63-related ectodermal dysplasias as well as for the treatment of patients with skin cancer.